Sextant ‘scopes for myopes.

26 11 2008

I have myopia, which is posh for being short sighted: anything more than 250 mm from my nose is blurred. I also have astigmatism, which means that objects that I can see are sharper along one meridian than any other. I have worn spectacles since I was thirteen. The lenses those many years ago were made of glass, which didn’t scratch easily, but now they are usually made of plastic for lightness of weight and often have an anti-reflective coating. My optician recently remarked on how scratched the right hand lens was compared to the left and I immediately thought of the optical instruments I use. The modern ones have plastic or rubber eye cups, but not the vintage and antique sextants. Blackened brass was good enough for the old seafarers.

When using a sextant, the myope has the option of removing his glasses, as the focussing arrangements can take care of his myopia, but that still leaves the astigmatism, which distorts and elongates images of stars. Facing page 37 of Peter Ifland’s wonderful book Taking the Stars (First edition, Malabar, Florida, 1998) is a photograph of a sextant adapted for use in freezing latifudes. The scope and magnifier eyepieces are covered with leather, to protect the skin from sticking to very cold metal surfaces. I have adapted the idea to protect my spectacle lenses. Here’s how to do it.

Cut out a  circle of wash leather (“chamois leather”) about 50 mm in diameter and punch a hole about 8 mm in diameter at its centre. I used a punch, but nail scissors might just cope with the task.

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Then capture the disc over the end of the telescope eyepiece using a cable tie pulled just tight enough to allow you to centralise the hole. Stretching the leather also enlarges the hole and we need to do this with the Galilean or “star” scopes, as we have to move our heads around in order to see the full field of view, rather like looking into a room through a round window. My initial 8 mm holes stretched to 12 mm.

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If your sextant is modern, you might at this point be content simply to tighten the cable tie and trim the end, but for an older sextant I would aim for authenticity. Wax some button thread with beeswax, take a few turns around the eyepiece to anchor the leather, knot and finish by trimming the thread and leather. The beeswax, as well as protecting the thread, also helps the knot to hold while you’re tying it.

The final result, on a Hughes and Son Galilean ‘scope from 1920, is shown below. Now I can steady the sextant against my glasses without fear of scratching them and have already applied the idea to the stereo microscope that I use to improve my seeing while restoring sextants and making clocks. It can also be adapted to binocular-type eyepieces, by unscrewing the eyecup and replacing it with the leather. Try it and experience the full field of view for the first time while wearing your spectacles. If you do remove the binocular eyecup though, protect the heads of the three little screws you will find underneath from corrosion, with a thin smear of waterproof marine grease.

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Heath Vernier Sextant Restored

23 11 2008

A couple of weeks ago on e-bay I acquired a Heath vernier sextant for what seemed to be very little. When it arrived from Hawaii, it was plain that it had spent the evening of its life hanging on a wall, as a goodly amount of Hawaian dust and a bit of copper wire came with it. It was homeless too. Sextants without cases are much cheaper than those that still have them. I often wonder what becomes of the cases and spare telescopes and eye shades. I have only once managed to buy a sextant telescope (by Hughes and Son) and I have a beautifully constructed rosewood case that once housed a Heath Pillar Sextant. It seems that when a sextant is put on display, the case and its other contents get forgotten and then lost. Thus, my Hawaian sextant needed a new case and an inverting telescope, but first it had to be restored.

A previous owner seemed to have believed that sextants look better without their paint, especially if the frame appears to be made of brass (they are made of a high-tin bronze). All paint had been stripped, probably by dipping the instrument in a bath of paint remover, as it had even disappeared from the backs of the mirrors. Everything was encrusted in dust and fluff, and the silver arc and vernier scale were black with tarnish, but fortunately nothing was broken. From its construction, it was obviously by Heath and Company, but the placard on the index arm expansion which should have said ‘”HEZZANITH”,Endless Tangent Screw Automatic Clamp’ was missing.  As this device was patented in 1909 and Heath and Co merged with W S Stanley in 1926, shortly after which most sextants sold were micrometer instruments, I could date the orphan fairly well.

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It seemed to breathe a sigh of relief as I carefully dismantled it down to the last screw and washer and cleaned each part individually in a soothing bath of 50% ammonia and dish-washing liquid. Screw heads had to have burrs removed and then be polished, moving parts degreased and paint applied where it would have been present in a brand new instrument. Usually, only the brass screw heads and the limb would be left unpainted, sometimes, too, the little protective covers for the mirror adjusting screws. Finally, it could be reassembled , with a smear of marine waterproof grease to moving parts, and the mirrors adjusted, with the result you see below.

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It took some patience to get the lenses of the telescope clean of dust and dirt. I invariably dismantle telescopes down to their component parts. It makes it much easier to get at the optics and also gives an opportunity to lightly oil the very fine threads that hold the lens mount in place (I’m thinking of that next restorer in 50 years time). I had to make the body of the inverting telescope myself from scratch, turning it  from a length of 1 inch (25.4 mm) round brass bar. I have a selection of lenses from junked optical instruments. Half of a pair of early 6 x 22 mm binoculars provided the lenses for this scope. I had to make a new eyepiece  and fit it to a length of 3/4 inch (19.05 mm) brass tube for a draw tube. It works well and there is no tell-tale blooming of the lenses to indicate that they are post-1950.

The case is made from my precious supply of solid African mahogany. A case of this period would have had comb joints at the corners, but these need special machinery to make, so I used box dovetails appropriate to the period. Pre-1900 cases usually had dovetailed corners, but with very fine pins (the bits that fit between the tails). I haven’t yet learned how to make these successfully, and in any case, they would be out of period. I carved the hook latches out of brass, using one from a Hughes and Son case as a template. The handle would have been of cast brass. Ones of the correct pattern are hard to find and expensive, so I am waiting for my income tax refund before stocking up on a few and buying some silver nitrate to use in re-silvering the mirrors.

You will find more about restoring in my book.





The USSR SNO-T sextant

22 11 2008

5 August 2011

I have now completed a manual on the structure, overhaul, repair, maintenance and use of the SNO-T sextant, with notes on the Freiberger Trommelsextant (drum sextant) where it differs significantly from the SNO-T. There are over 60 pages of detailed colour photographs and diagrams that will take you step by step through the overhaul of this fine instrument, down to the last nut, bolt and washer. It is available as a download of 5MB for US$19.50 or, for an extra US$4.00, on a CDROM posted to you anywhere in the world. To purchase, please send the correct amount via PayPal, with a note to remind me what it’s for, and use engineer@clear.net.nz as the payee.  You receive it as a pdf file attachment. Here are a couple of illustrations from the manual to whet your appetite:

Figure 1 : Index shades mounting.

Figure 2 : Step of dis-assembling micrometer mechanism.

Now, for a brief account of the SNO-T sextant, please read on.

The Navigational Sextant with Illumination, Tropicalised succeeded the SNO-M sextant in about 1976. While the SNO-M was a clone of the CPlath WW II sextant, the SNO-T was a modified copy of an early version of the Freiberger Prazisionsmechanik Trommelsextant, with the addition of a self-illuminating magnifier. The radius of the Trommelsextant is about 170 mm, as its worm has a pitch of 1.5 mm, while the SNO-T’s radius is 160 mm, the frame is more strongly braced and the worm is of 1.4 mm pitch. The final point of difference is that the Trommelsextant is a sextant, while the SNO-T is a quintant, reading up to 140 degrees.  A Freiberger employee told me the SNO-T was made in Leningrad (now St Petersburg), though the closeness of design makes it highly probable that the conception was German.

The Freiberger Trommelsextant was developed commercially in the 1950s and by the 80s was selling with a 3.5 X 40 telescope and a modified micrometer release catch position. The shades were also mounted on  to the edge of the frame rather than on the face as in the earlier instrument and the SNO-T.

The design of the SNO-T departed from the traditional in several respects. The most immediately noticeable is in the micrometer assembly, which is totally enclosed and closely embraces the rack. The release catch rotates the micrometer screw away from the rack by means of an eccentrically mounted bearing assembly and the pressure exerted on the rack by the worm can be adjusted to give silky-smooth rotation. The rack is machined directly into the frame. The very robust index arm lies behind the frame (i.e. on the right hand side) and so the handle has to be mounted on a sub-frame that bridges the index arm. Another radical departure from traditional design is in the form of the index arm bearing, which is a very substantial parallel bronze journal running directly in a hardened seat machined directly in the frame. The frame itself is of aluminium alloy. The edges are very substantial and braced by an elegant web on the face. A photoluminscent micrometer drum magnifier was provided, allowing easy estimation to tenths of a minute.

The 1976 specification claimed an instrumental accuracy of +/- 6 seconds with no more than 6 seconds backlash. A later specification modified these to 12 seconds.

The sextant was supplied with two telescopes of exceptional quality, probably by Zeiss. The inverting telescope, a 6 X 30, was identical to that supplied with the SNO-M, while the Galilean star ‘scope was a 4 X 40. There was also a kit of extras, comprising a spare (front-surface) index mirror, a cleaning brush, an oil bottle, two screwdrivers, two sighting vanes, a pin wrench to adjust the micrometer drum and a mirror adjusting wrench. The instrument itself weighed 1.5 kg. This very high quality sextant, possibly the best ever made, was let down by being contained in a grey-painted pinewood box and restrained in poorly engineered wooden pockets which were prone to disintegrate in transport. However, when the paint is stripped off and the wood stained and polished the result is very attractive.  Currently selling for around US$400, this represents to my mind a substantial undervaluing.

Many more details of the construction of this and other sextants may be found in my book, The Nautical Sextant .

Figure 3: SNO-T sextant, left hand face

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Figure 4 : SNO-T sextant, right hand face

1st July 2012

Robert Lawrence has kindly brought to my attention a sextant sold in 1995 by Sewill of Liverpool, a long-established firm of instrument makers. Although it is named Cetus Primus, it is plainly an SNO-T with some modifications. Figure 5 shows a finely-cut vernier for the micrometer that matches the style of the instrument, though anyone with a good quality dividing head could cut his own. While the magnifier could then be dispensed with, it has on its underside some photoluminescent paint that, once charged by exposure to a 40 watt lamp for a few minutes, gives about 40 minutes-worth of scale illumination.

Figure 5 :  Sewill Cetus Primus micrometer vernier

Figure 6 shows the horizon mirror of the same instrument. It has the so-called “full view” mirror that allows a full view of the horizon when observing the body. The inventors claimed that it reflects most of the light at the blue-yellow end of the spectrum (stars, sun, moon) and transmits most light at the red-orange end (twilit horizon). An experienced user said that it made easy sights easier and difficult sights harder.

Fitted to the mirror is a Davis prism attachment. This is in essence a narrow prism with an angle of about 6 degrees that allows a view of the horizon at one side of the mirror, off to one side of the rest of the horizon image. When the attachment is properly adjusted, if the two horizon images are brought into line, the frame of the sextant will be vertical and there will be no need to rock the sextant when taking sights. This would be of particular use when taking high altitude sights

Figure 6 : Sewill Cetus Primus “clear-view” mirror with Davis prism attachment.

 

Postscript, October 2013 The horizon shades of every SNO-T I have seen are mounted upside-down. See under “Blunders” category.





AIRCRAFT BUBBLE SEXTANTS

17 11 2008

I have recently been asked by a reader whether I plan to write a similar book about aircraft bubble sextants, such as the US A10 and A12 sextants of the Second World War. The answer is “perhaps when I have caught my breath after writing the Naked Nautical Sextant.”

However, readers may be interested in my Mark IXA Sextant – A reconditioning manual, also available as an e-book. The Mark IX series of bubble sextants served the Royal Air Force right through WW II and beyond. Thousands of these instruments survive and in the view of many, including me, it was the best designed of all the bubble sextants ever made. I have recently revised the manual and added a supplement on the Mark IX. If you’re interested, contact me.

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